1. Field of the Invention
The present invention relates to a phase plate, and more particularly to a phase plate having a fine periodic structure smaller than a visible light wavelength.
2. Description of the Related Art
Previously, as an optical element that gives a phase difference with respect to a transmission linear polarized light, a phase plate such as a ¼ wavelength plate or ½ wavelength plate has been known. Furthermore, there is a phase plate that has an optical axis in an arbitrary direction inclined from a surface normal direction. Such a phase plate is for example used in a projection image display apparatus to compensate a phase shift generated by a polarization beam splitter, an image display element, or the like, and therefore it has an effect that a light leakage in a projection image is reduced and that the contrast of the projection image is improved.
The phase plate is manufactured by using a fine structure having a period shorter than a wavelength of light to which a phase difference is provided and by controlling its period direction and shape. In this case, in order to obtain a phase plate in which an optical axis is inclined from a surface normal, the period direction itself of the structure may be inclined from a substrate surface. Japanese Patent Laid-Open No. 2004-163450 discloses a phase plate that uses a fine structure having a refractive index anisotropy equivalent to a structure where the optical axis is inclined from the surface normal at an arbitrary angle.
However, it is difficult to manufacture the fine structure as disclosed in Japanese Patent Laid-Open No. 2004-163450. Previously, as a method of fabricating such a fine structure, a method of using a nanoimprint in which a shape is transferred to a material using a mold having the fine periodic structure described above, and an etching process method using a fine mask have been known. However, since the process size is smaller than or equal to a wavelength and the periodic structure is inclined with respect to an element surface, it is difficult to release the mold after the molding in the method of the nanoimprint. Even when the etching is used, the shape needs to be fabricated in an oblique direction with respect to the substrate surface, and therefore it is difficult to uniformly process the oblique grating shape as disclosed in Japanese Patent Laid-Open No. 2004-163450.